The present exemplary embodiment relates to imaging systems in general. It finds particular application in conjunction with methods and systems for mounting an optical axis of an optical lens in an accurate desired alignment with an optical imager, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Increasingly, image sensing systems are being used for optical metrology (i.e., measurement) functions. For instance, image sensing systems may utilize various algorithms to calculate, inter alia, the angles, distances, curvatures, speeds, etc. of objects within an acquired image.
Such image sensing systems typically include a video camera, digital still camera or the like, which are capable of capturing images into digital signals (or potentially analog signals) for storage, manipulation and/or distribution. Such systems ordinarily include a lens or other image-forming element capable of capturing light from a scene/object and focusing/projecting that light onto a surface that is capable of sensing the light. This surface typically comprises an array of photo sensor elements, such as charge-coupled-devices (“CCDs”) or complementary metal oxide semiconductor (“CMOS”) photoreceptors.
These sensors typically comprise planar, rectangular matrices, or arrays, of photoelectric transducer elements fabricated on the surface of a semiconductor substrate, typically silicon, by various known photolithographic techniques, that are capable of converting the light energy incident upon them into electrical signals on an element-by-element, or pixel-by-pixel, basis. These signals, usually digital in nature, include information pertaining to, e.g., the intensity, color, hue, saturation, and other attributes of the incident light.
The sensor array substrate(s) is typically disposed on and electrically connected to a base substrate such as a printed circuit board (PCB). A lens structure disposed over the sensor array typically supports one or more optical elements. This lens structure typically mounts to the base substrate/PCB. The most common method for aligning the lens and optical sensor involves the lens and base substrate having corresponding, complementary mounting features adapted to engage each other such that, when engaged, the optical features of the sensor are aligned with the optical elements of the lens. Typically, the lens includes various projections arranged around its optical axis and the base substrate includes corresponding apertures or pedestals that mate with these projections.
In order to accurately align the lens and optical sensor, the tolerances of the individual components and the complementary mounting features need to be extremely tight. Maintaining such tight dimensional tolerances is difficult. Separate tolerances are present in the fabrication and assembly of the various components (e.g., optical properties of the lens elements, sensor to PCB mounting tolerances, lens to lens carrier mounting tolerances, etc.) and the final integration of these components (e.g., lens carrier to PCB mounting tolerances). Unless extremely tight dimensional tolerances are specified and maintained, during the fabrication and assembly of all of these components, tolerance stack ups tend to occur. Generally, such tolerance stack up can be maintained to within a few hundred microns in the Cartesian directions (e.g., XYZ directions) and to perhaps slightly more than 1° angularly about one or all of the Cartesian axes.
While such tolerances are acceptable for many applications, accurate optical metrology (i.e., measurement) functions typically require much tighter tolerances. To provide tighter tolerances, an active alignment process may be utilized where feedback from the optical sensor guides alignment. In such an active alignment method, the sensor is temporarily positioned loosely in about the desired position of alignment with the lens positioned loosely in about the desired position of alignment with the sensor. The sensor is temporarily connected to a display to output a test scene or pattern, and the relative position of the sensor or lens is adjusted by a human or machine until the image of the test pattern produced on the display subjectively matches the test pattern, whereupon the position of the sensor relative to the lens is then fixed permanently in place.
Active alignment typically requires multiple alignment and securing/gluing steps. For instance, one prior design utilizes a two piece lens mount carrier. The first piece is a lens mount with internal threads; the second piece is a mechanical stand-off for attachment to the PCB. First, the lens is focused by being threaded into or out of the lens mount in the Z direction. Then the lens and the lens mount assembly are moved in the XY directions on top of the mechanical stand-off to correct for any lateral misalignment. The process requires a 3-step gluing process that includes tacking the lens inside the threaded mount, tacking the lens mount on the stand-off, and removing the complete assembly from an alignment setup for the final gluing and re-enforcement.